Internal combustion engines are any of a group of devices in which the reactant of combustion, e.g., oxidizer and fuel, and the products of combustion serve as the working fluids of the engine. The basic components of an internal combustion engine are well known in the art and include the engine block, cylinders, pistons, valve, crankshaft and camshaft. The cylinder heads, cylinders and tops of the pistons typically form combustion chambers into which fuel and oxidizer (e.g. air) are introduced and combustion takes place. Such an engine gains its energy from the heat released during the combustion of the non-reacted working fluids, e.g., the oxidizer-fuel mixture. This process occurs within the engine and is part of the thermodynamic cycle of the device. In all internal combustion engines, useful work is generated from the hot, gaseous products of combustion acting directly on moving surfaces of the engine, such as the top or crown of a piston. Generally, reciprocating motion of the pistons is transferred to rotary motion of a crankshaft via connecting rods.
Internal combustion (IC) engines can be categorized into spark ignition (SI) and compression ignition (CI) categories. SI engines, i.e. typical gasoline engines, use a spark to ignite the air-fuel mixture, while the heat of compression ignites the air-fuel mixture in CI engines, i.e., typically diesel engines.
The most common internal combustion engine is the four-stroke cycle engine, a concept whose basic design has not changed for more than 100 years. This is because of its outstanding performance as a prime mover in the ground transportation industry. In a four-stroke cycle engine, power is recovered from the combustion process in four separate movements (strokes) of a single piston. For purposes herein, a stroke is defined as a complete movement of a piston from a top dead center position to a bottom dead center position or vice versa. Accordingly, a four-stroke cycle engine is defined herein to be an engine, which requires four complete strokes of one or more pistons for every power stroke, i.e. for every stroke that delivers power to a crankshaft.
Many rather exotic early engine designs were patented. Examples of these early patents include U.S. Pat. No. 2,091,413 of 1937 and U.S. Pat. No. 2,269,948 of 1942, both issued to M. Mallory. Various other relatively recent specialized prior art engines have also been designed in an attempt to increase engine efficiency, such as U.S. Pat. No. 5,546,897 issued in 1996 to D. Brackett, U.S. Pat. No. 5,623,894 issued in 1997 to J. Clarke, and U.S. Pat. No. 6,058,901 issued in 2000 to C. L. Lee. However, none were able to offer greater efficiencies or other significant advantages which would replace the standard engine.
Accordingly, there is a need for an improved four-stroke internal combustion engine which can enhance efficiency by more closely aligning the torque and force curves generated during a power stroke without increasing compression ratios substantially beyond normally accepted design limits.
The newcomer in the field of internal combustion engines is a split cycle engine which has been disclosed in a number of patents such as: U.S. Pat. No. 6,880,502 issued in 2005 to C. Scuderi, U.S. Pat. No. 7,017,536 issued in 2006 to C. Scuderi, U.S. Pat. No. 7,121,236 issued in 2006 to C. Scuderi, etc. The same engine has been patented in China, Taiwan, Japan, Korea and Russia. That invention offers advantages and alternatives over the prior art by providing a four-stroke cycle internal combustion engine having a pair of pistons in which one piston of the pair is used for the intake and compression strokes and another piston of the pair is used for the power and exhaust strokes, with each four-stroke cycle being completed in one revolution of the crankshaft. The engine enhances efficiency by more closely aligning the torque and force curves generated during a power stroke without increasing compression ratios.
These and other advantages are accomplished in an exemplary embodiment of the invention by providing a four-stroke split cycle internal combustion engine, which is shown in FIG. 1 in the form of a schematic diagram. The engine 10 comprises a crankshaft 12 journaled for rotation about a crankshaft axis 14 (extending perpendicular to the plane of the paper) of the engine 10. The crankshaft 12 of the engine 10 includes a first throw 16 and a second throw 18. A first connecting rod 20 is pivotally connected to both the first throw 16 of the crankshaft 12 and a power piston 22. The power piston 22 is slidingly received within a first cylinder 24, and being connected to the crankshaft 12, the power piston reciprocates through a power stroke and an exhaust stroke of a four-stroke cycle during a single rotation of the crankshaft. A second connecting rod 26 is pivotally connected to both the second throw 18 of the crankshaft 12 and a compression piston 26. The compression piston 28 is slidingly received within a second cylinder 30, and being connected to the crankshaft, the compression piston reciprocates through an intake stroke and a compression stroke of the same four stroke cycle during the same rotation of the crankshaft. The mechanical linkage of the connecting rods 16 and 18 to the pistons 22, 28 and crankshaft throws 16, 18 serve to convert reciprocating motion of the pistons (as indicated by directional arrow 48 for the power piston 22, and directional arrow 50 for the compression piston 28) to the rotary motion (as indicated by directional arrow 52) of the crankshaft 12. A gas passage 32 interconnects the first and second cylinders. The gas passage includes an inlet valve 34 and an outlet valve 36 defining a pressure chamber therebetween. The inlet valve permits substantially one way flow of compressed gas from the second cylinder to the pressure chamber and the outlet valve permits substantially one way flow of compressed gas from the pressure chamber to the first cylinder.
During operation the power piston 22 leads the compression piston 28 by a phase shift angle 38, defined by the degrees of rotation the crankshaft 12 must rotate after the power piston 22 has reached its top read center position in order for the compression piston 28 to reach its respective top dead center position. The above mentioned patents claim this phase shift angle to be between 20 and 60 degrees. For this particular embodiment the phase shift is fixed substantially at 50 degrees.
Because the compression and power strokes are performed by separate pistons 22 and 28, various enhancements can be made to optimize the efficiency of each stroke without the associated penalties incurred when the strokes are performed by a single piston. For example, the compression piston diameter 46 can be made larger than the power piston diameter 44 to further increase the efficiency of compression. Additionally, the radius 40 of the first throw 20 for the power piston 22 can be made larger than the radius 42 to further enhance the total torque applied to the crankshaft 12. FIG. 1 shows a modification of the embodiment of a split four-stroke engine having these unequal throws and unequal piston diameters. However, the schematics in a series of the aforementioned Scuderi engines, a typical representative of which is one disclosed, e.g., in aforementioned U.S. Pat. No. 6,880,502, depict and describe a crankshaft of a substantially conventional design which will be very difficult and quite expensive to manufacture and remanufacture for development, especially when a number of pairs of cylinders in an engine increases. A value of the power shift angle is not defined precisely in any of the above mentioned patents and is left to be determined during the development of an engine in a range between 20 and 60 degrees. A crankshaft of that kind will substantially increase cost of the development because a new crankshaft will be needed at each and every test. Balancing of that crankshaft will present a challenge as well. On the other hand, in U.S. Pat. No. 7,121,236 a crankshaft of a complicated design is described. It is questionable whether it will perform at a high rate of revolutions.